1. Field of the Invention
This invention generally relates to radiation detectors for monitoring the presence of radioactivity and particularly to a detector useful in monitoring radioactivity in a noisy environment.
2. Description of Related Art
A variety of radiation detectors have been developed for monitoring the presence of radioactive substances in commercial, laboratory and medical facilities. In these environments trained personnel handle radioactive substances and risk radiation exposure.
The standard for monitoring radiation is to determine any received dose in “rems.” A rem (radiation equivalent man) is related to a “rad” (radiation absorbed dose) based upon the nature and energy of the radioactivity. For gamma and beta radiation, one rad translates to one rem. For alpha radiation one rad translates to twenty rem; one rad of neutron radiation, to between five and twenty rem depending upon neutron energy. It is important to evaluate any radiation exposure in terms of rems because it is the most relevant measure of the biological effect of radiation exposure.
In many instances personnel in such environments wear radiation badges or carry pagers that monitor the absorbed dose in rem. In addition or as an alternative, trained personnel use Geiger counters. Geiger counters use a gas tube radiation detector and emit clicks when high energy radioactive particles are detected. However, Geiger counters are not sensitive to very low levels of radiation and have no ability to distinguish the energy of a particle that causes a click. Geiger counters typically provide an indication of radiation exposure in rads, not rems.
U.S. Pat. No. 2,968,726 (1961) to Bersin et al. discloses a radiation measuring instrument for measuring the effects of neutron radiation to 10 Mev and gamma radiation. A circuit including an ion chamber and amplifier determine the effects of the gamma and low energy neutron radiation, e.g., less than 250 Kev. A methane counter, pulse amplifier and discriminator produce pulses that contain information about all neutron energies above the 250 Kev. In order to maintain linear operation of the circuitry, Bersin et al. incorporate a pulse stretcher to attenuate incoming pulses and offset the attenuation by stretching the pulse width. An amplifier combines the processed outputs of the ion chamber and the methane counter to obtain rem values.
FIG. 1 is a simplified block diagram that depicts another example of a radiation detection apparatus that generates rem readings. This is an XRF ICS-400 spectrometer available from the assignee of this invention. Specifically this radiation detection apparatus 10 includes a detector 11 that generates an output pulse for each interaction with a radioactive particle, i.e., an “interaction event.” The detector 11 can comprise a scintillating crystal that generates a light pulse, or “flash” in response to each interaction event. A photodiode or photomultiplier detector circuit 11 converts each light pulse into an electrical pulse, Vin. Alternatively crystals of cadmium telluride or similar materials can be substituted to convert radioactive particles directly into current if biased with a dc voltage.
In whatever form, each pulse is a low level signal with only a few thousand electrons and an amplitude that varies as the energy of the incident particle and that typically has a time duration in the microsecond region. That is, the electrical pulse Vin has an amplitude that represents the energy level for the radioactive particle that caused the interaction event. All Vin pulses can be considered to be “essentially instantaneous” when analyzed in terms of millisecond or second time domains.
Still referring to FIG. 1, an amplified unipolar pulse from a preamplifier 13 passes into a band pass filter 14 including a low pass filter 15 and high pass filter 16. In one embodiment, this amplified unipolar pulse has a duration of about 3 microseconds and an amplitude of about 1 mV per 1 KeV of energy. The band pass filter 14 shapes the incoming signals to reject high and low frequency noise usually by being constructed to produce a Gaussian-shaped pulse or an output that represents the output of the filter with a triangular response for amplitude versus frequency. The band pass filter 14 produces filtered output pulses Vf. A comparator 17 receives the Vf output pulses at its negative input and a dc bias signal from a dc bias source 20 at its positive input.
That portion of each Vf pulse that has an amplitude greater than the dc bias passes to a low pass filter 21 as output voltage V0. The V0 signal represents the product of the energy of each pulse and the pulses per second. Thus, the output approximately measures the danger of radiation by calculating the product of pulse energy and the number of pulses/second. The inclusion of pulse energy is preferred because high energy particles do more damage to human body.
FIG. 1 and U.S. Pat. No. 2,968,726 disclose radiation detectors that provide appropriate results in “clean” or “low-noise” environments. However, certain new requirements for radiation detection do not have the luxury of operating in a low-noise environment. For example, there is now a threat of terrorist-sponsored attacks using radioactive substances. In the event any attack in which the dispersal of radioactive substances is possible, first responders must determine if a location is contaminated and if so, must determine the level of radioactivity accurately. In this application, the range of energy levels of interest is 50 KeV to 2 Mev.
As first responders or others approach the area of such an attack, they will encounter high levels of acoustic noise from fire sirens and air horns that emit high levels of acoustic broadband sound or noise. It has been found that prior art detection apparatus in the vicinity will respond to such sounds or noise as well as to interaction events. That is, the prior art detection apparatus will detect noise as if radiation were present when it is not. For example, detectors that use high voltages to bias the crystal can generate current in response to any mechanical motion of the detector following the capacitance equation Q=dc*V where q is the resultant current in coulombs when a capacitance biased with V volts is displaced with the resultant capacitance change of dc. This acoustic energy is difficult to shield because sirens, for example, have acoustic energy reaching past 1 MHz. The amplitude requires approximately 60 db of acoustic isolation to protect the prior art detectors from such acoustic noise.
For enhanced personnel protection, such radiation detectors should be mounted on the exterior of a vehicle so that personnel remain in the vehicle until it is determined to be safe to exit the vehicle. However, an externally mounted detector is subject to being struck by stones and other road debris. Such impact events can cause even larger broadband noise pulses that may last for 5-10 mS. These noise pulses when generated are a problem because they are categorized as radiation pulses or interaction events. Thus prior art radiation detectors record both the signals generated by noise as well as signals induced by interaction events. Consequently the noise readings in such detectors can overstate the level of radiation that is present.
It will be necessary to detect low levels of radiation when approaching such a location. Prior art devices for detecting low levels of radioactivity can be expensive. The application of these radiation detectors to first responder vehicles and like applications will require large numbers of devices. The costs may be prohibitive.
Therefore what is needed is a radiation detector that can detect a wide range of radiation levels in a noisy environment, that is adapted for mounting to a vehicle and that is less expensive to manufacture than prior art radiation devices.